I. Field
The present invention relates generally to communication, and more specifically to techniques for transmitting data in a wireless communication system.
II. Background
Wireless communication systems are widely deployed to provide various communication services such as voice, packet data, multi-media broadcast, text messaging, and so on. For example, a base station may transmit user-specific data to certain terminals and may broadcast multi-media data to all terminals. The terminals may be distributed throughout the coverage area of the base station. Each terminal observes a different wireless channel between that terminal and the base station. The wireless channels for the terminals may experience different channel conditions (e.g., different fading, multipath, and interference effects) and may achieve different signal-to-noise-and-interference ratios (SNRs). The SNR of a wireless channel determines its transmission capacity, which is typically quantified by a particular data rate that may be reliably transmitted via the wireless channel.
A user-specific transmission is a data transmission sent to a specific terminal. A user-specific transmission is typically encoded and transmitted in a manner such that the recipient terminal can reliably receive the transmission. This is often achieved by estimating the SNR of the wireless channel for the terminal and encoding the transmission based on the SNR estimate.
A broadcast transmission is a data transmission sent to a group of terminals or to all terminals. A broadcast transmission is typically encoded and transmitted in a manner to achieve a specified quality of service (QoS). This quality of service may be quantified, for example, by error free reception of the broadcast transmission by a specified percentage (e.g., 95%) of the terminals within a broadcast coverage area. Equivalently, the quality of service may be quantified by an outage probability, which is the percentage of the terminals within the broadcast coverage area that cannot correctly decode the broadcast transmission.
A broadcast transmission observes different wireless channels for different terminals in the broadcast coverage area. The wireless channel for each terminal may be random with respect to the wireless channels for the other terminals. Furthermore, the wireless channels for the terminals may vary over time. To ensure that the broadcast transmission can meet the specified quality of service, the data rate for the broadcast transmission is typically selected to be sufficiently low and the coding and modulation for the broadcast transmission are typically selected to be sufficiently robust so that the broadcast transmission can be reliably decoded even by the terminal with the worst channel conditions. The broadcast performance for such a system would then be dictated by the worst-case channel conditions for all of the terminals in the broadcast coverage area.
There is therefore a need in the art for techniques to more efficiently broadcast data in a wireless communication system.